This invention pertains to a process and catalyst for the direct oxidation of olefins, such as propylene, by oxygen to olefin oxides, such as propylene oxide.
Olefin oxides, such as propylene oxide, are used to alkoxylate alcohols to form polyether polyols, such as polypropylene polyether polyols, which find significant utility in the manufacture of polyurethanes and synthetic elastomers. Olefin oxides are also important intermediates in the manufacture of alkylene glycols, such as propylene glycol and dipropylene glycol, and alkanolamines, such as isopropanolamine, which are useful as solvents and surfactants.
Propylene oxide is produced commercially via the well-known chlorohydrin process wherein propylene is reacted with an aqueous solution of chlorine to produce a mixture of propylene chlorohydrins. The chlorohydrins are dehydrochlorinated with an excess of alkali to produce propylene oxide. This process suffers from the production of a low concentration salt stream. (See K. Weissermel and H. J. Arpe, Industrial Organic Chemistry, 2nd ed., VCH Publishers, Inc., New York, N.Y., 1993, p. 264-265.)
Another well-known route to olefin oxides relies on the transfer of an oxygen atom from an organic hydroperoxide or peroxycarboxylic acid to an olefin. In the first step of this oxidation route, a peroxide generator, such as isobutane or acetaldehyde, is autoxidized with oxygen to form a peroxy compound, such as t-butyl hydroperoxide or peracetic acid. This compound is used to epoxidize the olefin, typically in the presence of a transition metal catalyst, including titanium, vanadium, molybdenum, and other heavy metal compounds or complexes. Along with the olefin oxide produced, this process. disadvantageously produces equimolar amounts of a coproduct, for example an alcohol, such as t-butanol, or an acid, such as acetic acid, whose value must be captured in the market place. (Industrial Organic Chemistry, ibid., p. 265-269.)
Metal-catalyzed processes for the direct oxidation of propylene by oxygen are known. For example, U.S. Pat. No. 5,525,741 discloses the direct oxidation of propylene with oxygen in the presence of a crystalline metallosilicate, such as titanosilicate, having supported thereon a silver salt of nitric or nitrous acid. This patent is silent with respect to conducting the process in the presence of hydrogen.
PCT publication WO-A1-96/02323 discloses the hydro-oxidation of an olefin, including propylene, with oxygen in the presence of hydrogen and a catalyst to form an olefin oxide. The catalyst is a titanium or vanadium silicalite containing at least one platinum group metal, and optionally, an additional metal selected from silver, iron, cobalt, nickel, rhenium, and gold.
The aforementioned direct oxidations employ catalysts which are deficient in activity and/or selectivity to propylene oxide.
PCT publication WO-A1-97/25143 discloses the hydro-oxidation of an olefin, including propylene, with oxygen in the presence of hydrogen and a catalyst to form the corresponding olefin oxide. The catalyst is a titanium or vanadium silicalite containing a lanthanide metal. Optionally, an additional metal selected from the Group 8 metals of the Periodic Table, rhemnum, silver, and gold may be incorporated into the catalyst. Catalysts consisting of a lanthanide metal and a titanium or vanadium silicate exhibit low activity to propylene oxide.
In view of the above, a need continues to exist in the chemical industry for an efficient direct route to propylene oxide and higher olefin oxides from the reaction of oxygen with C3 and higher olefins. The discovery of such a process which simultaneously achieves high selectivity to the olefin oxide at an economically advantageous conversion of the olefin would represent a significant achievement over the prior art.
This invention is a novel process of preparing an olefin oxide directly from an olefin and oxygen and hydrogen. The process comprises contacting an olefin having at least three carbon atoms with oxygen in the presence of hydrogen and a catalyst under process conditions sufficient to produce the corresponding olefin oxide. The catalyst which is employed in the process of this invention comprises silver and titanium. In another aspect of the process of this invention, the catalyst comprising silver and titanium can further comprise gold, or at least one promoter element as noted hereinafter, or a combination of gold with one or more promoter elements.
The novel process of this invention is useful for producing an olefin oxide directly from oxygen and hydrogen and an olefin having three or more carbon atoms. Under preferred process conditions, the olefin oxide is produced in a high selectivity at a good conversion of the olefin.
In another aspect, this invention is a unique catalyst composition comprising silver, at least one promoter element, and a titanium-containing support. The promoter element is selected from Group 1, Group 2, zinc, cadmium, the rare earth lanthanides, and the actinide elements, as well as combinations of these elements.
In a third aspect, this invention is a unique catalyst composition comprising silver, gold, and a titanium-containing support. Optionally, this catalyst can contain at least one promoter element selected from Group 1, Group 2, zinc, cadmium, the rare earth lanthanides, and the actinide elements, including combinations thereof.
The novel compositions of this invention can be effectively used in the aforementioned direct oxidation of an olefin having three or more carbon atoms to the corresponding epoxide. In preferred embodiments, the catalysts achieve a high selectivity to olefin oxide at a good conversion of the olefin. When the catalyst is partially or completely spent, it is easy to regenerate. Accordingly, this composition possesses desirable properties for catalyzing the direct oxidation of propylene and higher olefins to their corresponding olefin oxides.
The novel process of this invention comprises contacting an olefin having three or more carbon atoms with oxygen in the presence of hydrogen and an epoxidation catalyst under process conditions sufficient to prepare the corresponding olefin oxide. In one preferred embodiment, a diluent is employed, as described in detail hereinafter. The relative molar quantities of olefin, oxygen, hydrogen, and optional diluent can be any which are sufficient to prepare the desired olefin oxide. In a preferred embodiment of this invention, the olefin employed is a C3-12 olefin, and it is converted to the corresponding C3-12 olefin oxide. In a more preferred embodiment, the olefin is a C3-8 olefin, and it is converted to the corresponding C3-8 olefin oxide. In a most preferred embodiment, the olefin is propylene, and the olefin oxide is propylene oxide.
The catalyst employed in the aforementioned process of this invention comprises silver and titanium. In a preferred embodiment, the catalyst comprising silver and titanium is essentially free of the Group 8 metals. The Group 8 metals include iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum. The term xe2x80x9cessentially free,xe2x80x9d as used in this context, means that the total concentration of these metals is less than about 0.01 weight percent, preferably, less than about 0.005 weight percent, based on the total weight of the catalyst composition.
In another preferred embodiment, the catalyst comprises silver, gold, and a titanium-containing support. This catalyst embodiment is also essentially free of the Group 8 metals, as defined hereinbefore.
In yet another preferred embodiment, the catalyst comprises silver and at least one promoter element on a titanium-containing support. The promoter is selected from Group 1, Group 2, zinc, cadmium, the rare earth lanthanides, and the actinides of the Periodic Table of the Elements, as referenced in the CRC Handbook of Chemistry and Physics, 75th edition, CRC Press, 1994-1995. Combinations of the aforementioned promoters can also be employed. In a more preferred embodiment, the support excludes a Group 2 metal titanate. In still another preferred embodiment, the catalyst comprises silver, gold, and at least one promoter selected from Group 1, Group 2, zinc, cadmium, the rare earth lanthanides, and the actinide elements, on a titanium-containing support.
Any olefin containing three or more carbon atoms can be employed in the process of this invention. Monoolefins are preferred, but compounds containing two or more carbonxe2x80x94carbon double bonds, such as dienes, can also be used. The olefin can be a simple hydrocarbon containing only carbon and hydrogen atoms; or alternatively, the olefin can be substituted at any of the carbon atoms with an inert substituent. The term xe2x80x9cinertxe2x80x9d, as used herein, requires the substituent to be substantially non-reactive in the process of this invention. Suitable inert substituents include, but are not limited to, halides, ether, ester, alcohol, and aromatic moieties, preferably chloro, C1-12 ether, ester, and alcohol moieties and C6-12 aromatic moieties. Non-limiting examples of olefins which are suitable for the process of this invention include propylene, 1-butene, 2-butene, 2-methylpropene, 1-pentene, 2-pentene, 2-methyl-1-butene, 2-methyl-2-butene, 1-hexene, 2-hexene, 3-hexene, and analogously, the various isomers of methylpentene, ethylbutene, heptene, methylhexene, ethylpentene, propylbutene, the octenes, including preferably 1-octene, and other higher analogues of these; as well as butadiene, cyclopentadiene, dicyclopentadiene, styrene, (xcex1-methylstyrene, divinylbenzene, allyl chloride, allyl alcohol, allyl ether, allyl ethyl ether, allyl butyrate, allyl acetate, allyl benzene, allyl phenyl ether, allyl propyl ether, and allyl anisole. Preferably, the olefin is an unsubstituted or substituted C3-12 olefin, more preferably, an unsubstituted or substituted C3-8 olefin. Most preferably, the olefin is propylene. Many of the aforementioned olefins are available commercially; others can be prepared by chemical processes known to those skilled in the art.
The quantity of olefin employed in the process can vary over a wide range provided that the corresponding olefin oxide is produced. Generally, the quantity of olefin depends upon the specific process features, including for example, the design of the reactor, the specific, olefin, and economic and safety considerations. Those skilled in the art will know how to determine a suitable range of olefin concentrations for the specific process features. Typically, on a molar basis an excess of olefin is used relative to the oxygen, because this condition enhances the productivity to olefin oxide. In light of the disclosure herein, the quantity of olefin is typically greater than about 1, preferably, greater than about 10, and more preferably, greater than about 20 mole percent, based on the total moles of olefin, oxygen, hydrogen, and optional diluent. Typically, the quantity of olefin is less than about 99, preferably, less than about 85, and more preferably, less than about 70 mole percent, based on the total moles of olefin, oxygen, hydrogen, and optional diluent.
Oxygen is also required for the process of this invention. Any source of oxygen is acceptable, including air and essentially pure molecular oxygen. Other sources of oxygen may be suitable, including ozone and nitrogen oxides, such as nitrous oxide. Molecular oxygen is preferred. The quantity of oxygen employed can vary over a wide range provided that the quantity is sufficient for producing the desired olefin oxide. Ordinarily, the number of moles of oxygen per mole of olefin is less than 1. Under these conditions the selectivity to olefin oxide is enhanced while the selectivity to combustion products, such as carbon dioxide, is minimized. Preferably, the quantity of oxygen is greater than about 0.01, more preferably, greater than about 1, and most preferably greater than about 5 mole percent, based on the total moles of olefin, hydrogen, oxygen, and optional diluent. Preferably, the quantity of oxygen is less than about 30, more preferably, less than about 25, and most preferably less than about 20 mole percent, based on the total moles of olefin, hydrogen, oxygen, and optional diluent. Above about 20 mole percent, the concentration of oxygen may fall within the flammable range for olefin-hydrogen-oxygen mixtures.
Hydrogen is also required for the process of this invention. In the absence of hydrogen, the activity of the catalyst is significantly decreased. Any source of hydrogen can be used in the process of this invention including for example, molecular hydrogen obtained from the dehydrogenation of alkanes and alcohols. In an alternative embodiment, the hydrogen may be generated in situ in the olefin oxidation process, for example, by dehydrogenating alkalies, such as propane or isobutane, or alcohols, such as isobutanol. Alternatively, hydrogen can be used to generate a catalyst-hydride complex or a catalyst-hydrogen complex which can supply the necessary hydrogen to the process.
Any quantity of hydrogen can be employed in the process provided that the amount is sufficient to produce the olefin oxide. Suitable quantities of hydrogen are typically greater than about 0.01, preferably, greater than about 0. 1, and more preferably, greater than about 3 mole percent, based on the total moles of olefin, hydrogen, oxygen, and optional diluent. Suitable quantities of hydrogen are typically less than about 50, preferably, less than about 30, and more preferably, less than about 20 mole percent, based on the total moles of olefin, hydrogen, oxygen, and optional diluent.
In addition to the above reagents, it may be desirable to employ a diluent in the reaction mixture, although the use thereof is optional. Since the process of this invention is exothermic, a diluent beneficially provides a means of removing and dissipating the heat produced. In addition, the diluent provides an expanded concentration regime in which the reactants are non-flammable. The diluent can be any gas or liquid which does not inhibit the process of this invention. The specific diluent chosen will depend upon the manner in which the process is conducted. For example, if the process is conducted in a gas phase, then suitable gaseous diluents include, but are not limited to, helium, nitrogen, argon, methane, carbon dioxide, steam, and mixtures thereof. Most of these gases are essentially inert with respect to the process of this invention. Carbon dioxide and steam may not necessarily be inert, but may have a beneficial promoting effect. If the process is conducted in a liquid phase, then the diluent can be any oxidation stable and thermally stable liquid. Suitable liquid diluents include aromatics, such as benzene; chlorinated aromatics, such as chlorobenzene and dichlorobenzene; aliphatic alcohols, such as methanol; chlorinated aliphatic alcohols, such as chloropropanol; as well as liquid polyethers, polyesters, and polyalcohols.
If used, the amount of diluent is typically greater than about 0.01, preferably greater than about 0.1, and more preferably, greater than about 15 mole percent, based on the total moles of olefin, oxygen, hydrogen, and optional diluent. The amount of diluent is typically less than about 90, preferably, less than about 80, and more preferably, less than about 70 mole percent, based on the total moles of olefin, oxygen, hydrogen, and diluent.
The concentrations of olefin, oxygen, hydrogen, and diluent disclosed hereinabove are suitably based on the reactor designs and process parameters disclosed herein. Those skilled in the art will recognize that concentrations other than those disclosed herein may be suitably employed in other various engineering realizations of the process.
Unique catalysts which are beneficially employed in the process of this invention comprise silver and titanium. The silver can exist as individual atoms and/or in discrete silver particles and/or, if a promoter is used, in mixed silver-promoter particles. The formal oxidation state of the silver can be any oxidation state or combination of states that provides for an active catalyst.
In another aspect, unique catalysts which are beneficially employed in the process of this invention comprise silver and gold and titanium. The silver can exist as individual atoms and/or in discrete silver particles and/or in silver-gold particles and/or, if a promoter is used, in mixed silver-gold-promoter particles. The formal oxidation state of the silver and/or the gold can be any oxidation state or combination of states that provides for an active catalyst.
The titanium preferably is present as a titanium-containing support which may take a variety of forms. The titanium predominantly exists in a positive oxidation state, as determined by X-ray photoelectron and X-ray absorption spectroscopies. More preferably, the titanium exists in an oxidation state of about +2 or higher, most preferably, in an oxidation state of about +3 to about +4. Non-limiting examples of titanium-containing supports which can be suitably employed in the catalyst of this invention include those described hereinbelow. Titanium-containing supports noted hereinbelow which do not contain the desired promoter element(s) must be treated to incorporate the promoter(s) into or onto the support. Supports already containing promoters may or may not require extra promoter element(s) to be added to the support.
a. Titanium Dioxide
Amorphous and crystalline titanium dioxide can be suitably employed as the titanium-containing support. The crystalline phases include anatase, rutile, and brookite. Included in this category are composites comprising titanium dioxide supported on silica, alumina, aluminosilicates, or other supports or combinations of supports.
The titanium dioxide may be deposited on the support in a number of methods. One example of the preparation which may be used herein is given by M. Haruta et al. in the European Patent Application EP 0709360A1, incorporated herein by reference. More generally, the support may be calcined in air to a temperature between 50xc2x0 C. and 800xc2x0 C. prior to the deposition of the titanium compound. The support is then impregnated with a titanium compound which is poorly reactive with the surface hydroxyls on the support. Typically, a solution containing the titanium compound is contacted with the support under mild conditions, such as a temperature between about 0xc2x0 C. and about 50xc2x0 C., at about ambient pressure for a time ranging from about 30 minutes to about 24 hours. A non-limiting example of a suitable titanium compound includes titanium oxide acetylacetonate or titanyl acetylacetonate. The solvent can be any which solubilizes the titanium compound, for example, aliphatic alcohols or aliphatic and aromatic hydrocarbons. After contacting the support with the solution containing the titanium compound, the support is dried at a temperature between about 0xc2x0 C. and about 150xc2x0 C., preferably between about 50xc2x0 C. and about 150xc2x0 C., in a vacuum or in a stream of air or an inert gas, such as nitrogen, argon, or helium. Thereafter, the support can be calcined in air to a temperature between about 300xc2x0 C. and about 800xc2x0 C., preferably between about 400xc2x0 C. and about 650xc2x0 C.
b. Promoter Metal Titanates
Stoichiometric and non-stoichiometric compounds comprising promoter metal titanates can also be suitably employed as the catalyst support. The promoter metal titanates can be crystalline or amorphous. Non-limiting examples of these include the titanates of Group 1, Group 2, and the lanthanide and actinide metals. Preferably, the promoter metal titanate is selected from the group consisting of magnesium titanate, calcium titanate, barium titanates, strontium titanate, sodium titanate, potassium titanate, and the titanates of erbium, lutetium, thorium, and uranium.
c. Titanosilicates
Crystalline and amorphous titanosilicates, preferably those that are porous, are also suitably employed as the support. Titanosilicates can be microporous materials incorporating Ti in the structure; these may be zeolitic materials. Within the framework structure of porous titanosilicates there exists a regular or irregular system of pores and/or channels. Empty cavities, referred to as cages, can also be present. The pores can be isolated or interconnecting, and they can be one, two, or three dimensional. The pores are more preferably micropores or mesopores or some combination thereof. As used herein, a micropore has a pore diameter (or critical dimension as in the case of a non-circular perpendicular cross-section) ranging from about 4 xc3x85 to about 20 xc3x85, while a mesopore has a pore diameter or critical dimension ranging from greater than about 20 xc3x85 to about 500 xc3x85. The combined volume of the micropores and the mesopores preferably comprises about 70 percent or greater of the total pore volume, and more preferably, about 80 percent or greater of the total pore volume. The balance of the pore volume will comprise macropores, which have a pore diameter of greater than about 500 xc3x85. Macropores include the void spaces between particles or crystallites.
The pore diameter (or critical dimension), pore size distribution, and surface area of the porous titanosilicate can be obtained from the measurement of adsorption isotherms and pore volume. Typically, the measurements are made on the titanosilicate in powder form using as an adsorbate nitrogen at 77 K or argon at 88 K and using any suitable adsorption analyzer, such as a Micromeritics ASAP 2000 instrument. Measurement of micropore volume is derived from the adsorption volume of pores having a diameter in the range from about 4 xc3x85 to about 20 xc3x85. Likewise, measurement of mesopore volume is derived from the adsorption volume of pores having a diameter in the range from greater than about 20 xc3x85 to about 500 xc3x85. From the shape of the adsorption isotherm, a qualitative identification of the type of porosity, for example, microporous or macroporous, can be made. Additionally, increased porosity can be correlated with increased surface area. Pore diameter (or critical dimension) can be calculated from the data using equations described by Charles N. Satterfield in Heterogeneous Catalysis in Practice, McGraw-Hill Book Company, New York, 1980, pp. 106-114, incorporated herein by reference.
Additionally, crystalline porous titanosilicates can be identified by X-ray diffraction methods (XRD), either by comparing the XRD pattern of the material of interest with a previously published standard or by analyzing the XRD pattern of a single crystal to determine framework structure, and if pores are present, the pore geometry and pore size.
Non-limiting examples of porous titanosilicates which are suitably employed in the process of this invention include porous amorphous titanosilicates; porous layered titanosilicates; crystalline microporous titanosilicates, such as titanium silicalite-1 (TS-1), titanium silicalite-2 (TS-2), titanosilicate beta (Ti-beta), titanosilicate ZSM-12 (Ti-ZSM-12) and titanosilicate ZSM-48 (Ti-ZSM-48); as well as mesoporous titanosilicates, such as Ti-MCM-41.
Titanium silicalite and its characteristic XRD pattern have been reported in U.S. Pat. No. 4,410,501, incorporated herein by reference. TS-1 can be obtained commercially, but it can also be synthesized following the methods described in U.S. Pat. No. 4,410,501. Other preparations have been reported by the following (incorporated herein by reference): A. Tuel Zeolites, 1996, 16, 108-117; by S. Gontier and A. Tuel, Zeolites, 1996, 16, 184-195; by A. Tuel and Y. Ben Taarit in Zeolites, 1993, 13, 357-364; by A. Tuel, Y. Ben Taarit and C. Naccache in Zeolites, 1993, 13, 454-461; by A. Tuel and Y. Ben Taarit in Zeolites, 1994, 14, 272-281; and by A. Tuel and Y. Ben Taarit in Microporous Materials, 1993, 1, 179-189.
TS-2 can be synthesized by the methods described in the following references (incorporated herein by reference): J. Sudhakar Reddy and R. Kumar, Zeolites, 1992, 12, 95-100; by J. Sudhakar Reddy and R. Kumar, Journal of Catalysis, 1991, 130, 440-446; and by A. Tuel and Y. Ben Taarit, Applied Catal. A, General, 1993, 102, 69-77.
The structure and preparation of titanosilicate beta have been described in the following references, incorporated herein by reference: PCT patent publication WO 94/02245 (1994); M. A. Camblor, A. Corma, and J.H. Perez-Pariente, Zeolites, 1993, 13, 82-87; and M. S. Rigutto, R. de Ruiter, J. P. M. Niederer, and H. van Bekkum, Stud. Surf. Sci. Cat., 1994, 84, 2245-2251.
The preparation and structure of Ti-ZSM-12 are described by S. Gontier and A. Tuel, ibid., incorporated herein by reference.
References to the preparation and structure of Ti-ZSM-48 include R. Szostak, Handbook of Molecular Sieves, Chapman and Hall, New York, 1992, p. 551-553; as well as C. B. Dartt, C. B. Khouw, H. X. Li, and M. E. Davis, Microporous Materials, 1994, 2 425-437; and A. Tuel and Y. Ben Taarit, Zeolites, 1996, 15, 164-170. The aforementioned references are incorporated herein by reference.
Ti-MCM-41, its structure, and preparation are described in the following citations incorporated herein by reference: S. Gontier and A. Tuel, Zeolites, 1996, 15, 601-610; and M. D. Alba, Z. Luan, and J. Klinowski, J. Phys. Chem., 1996, 100, 2178-2182.
The silicon to titanium atomic ratio (Si/Ti) of the titanosilicate can be any ratio which provides for an active and selective epoxidation catalyst in the process of this invention. A generally advantageous Si/Ti atomic ratio is equal to or greater than about 5/1, preferably, equal to or greater than about 10/1. A generally advantageous Si/Ti atomic ratio is equal to or less than about 200/1, preferably, equal to or less than about 100/1. It is noted that the Si/Ti atomic ratio defined herein refers to a bulk ratio which includes the total of the framework titanium and the extra-framework titanium. At high Si/Ti ratios, for example, about 100/1 or more, there may be little extra-framework titanium and the bulk ratio essentially corresponds to the framework ratio.
d. Titanium Dispersed on a Support
Another suitable support for the catalyst of this invention comprises titanium dispersed on a support such as silica, alumina, aluminosilicates, or any other support or combinations of supports. This support can be obtained commercially, or alternatively, prepared by the methods described hereinbelow.
In the aforementioned support, the titanium ions are dispersed over the surface of the silica substantially in a disorganized phase. The titanium ions in the disorganized phase may be isolated from other titanium ions, or alternatively, the titanium ions may be linked through oxide bonds to other titanium ions in small domains of a two-dimensional monolayer network. Whatever its actual topology, the disorganized phase does not exhibit an organized, periodic crystallinity. The disorganized phase can be distinguished from a bulk organized phase by one or more modern analytical techniques, for example, high resolution transmission electron microscopy and Raman spectroscopy. Ultraviolet visible diffuse reflectance spectroscopy and titanium K edge X-ray absorption near edge structure spectroscopy may also be useful. These techniques and others are known to those skilled in the art.
Any silica can be used in the support provided that it allows for an active catalyst composition. The silicas can be amorphous or crystalline. Preferred silicas are surface hydroxylated. Non-limiting examples of suitable silicas include fumed silica, silica gel, precipitated silicas, precipitated silica gels, silicalite, and mixtures thereof. Preferably, the surface area of the silica is, greater than about 15 m2/g, more preferably, greater than about 20 m2/g, and most preferably, greater than about 25 m2/g. More preferably, the surface area of the silica is less than about 800 m2/g, most preferably, less than about 600 m2/g.
Any alumina can be used in the support provided that it allows for an active catalyst composition. The aluminas can be amorphous or crystalline. Preferred aluminas are surface hydroxylated. Preferably, the surface area of the alumina is greater than about 15 m2/g, more preferably, greater than about 20 m2/g and most preferably, greater than about 25 m2/g. More preferably, the surface area of the alumina is less than about 800 m2/g, most preferably, less than about 600 m2/g.
The titanium loading on the support can be any which gives rise to an active catalyst in the process of this invention. Typically, the titanium loading is greater than about 0.02 weight, percent, preferably, greater than about 0.1 weight percent, based on the weight of the support. Typically, the titanium loading is less than about 20 weight percent, and preferably less than about 10 weight percent, based on the weight of the support.
The method of depositing the titanium ions on the support is important in obtaining the disorganized titanium phase described hereinabove. A description along the lines of the preparation used herein is given by S. Srinivasan et al. in the Journal of Catalysis, 131, 260-275 (1991), and by R. Castillo et al., Journal of Catalysis, 161, 524-529 (1996), incorporated herein by reference. Generally, the support is impregnated with a titanium compound which is reactive with the surface hydroxyls on the support. Typically, a solution containing a reactive titanium compound is contacted with the silica under mild conditions, such as a temperature between about 0xc2x0 C. and about 50xc2x0 C., at about ambient pressure for a time ranging from about 30 minutes to about 24 hours. Non-limiting examples of suitably reactive titanium compounds include titanium alkoxides, such as titanium isopropoxide, titanium propoxide, titanium ethoxide, and titanium butoxide; titanium sulfate, titanium oxysulfate, titanium halides, preferably titanium chloride; titanium carboxylates, preferably titanium oxalate; and organotitanium halides, such as dicyclopentadiene titanium dichloride, and other organotitanocene dichllorides. Preferably, titanium alkoxides are employed. The solvent can be any which solubilizes the reactive titanium compound, for example, aliphatic alcohols, aliphatic and aromatic hydrocarbons, and water where appropriate. After contacting the support with the solution containing the reactive titanium compound, the support is dried at a temperature between about 0xc2x0 C. and about 150xc2x0 C., preferably between about 50xc2x0 C. and about 150xc2x0 C., in a vacuum or in a stream of air or an inert gas, such as nitrogen, argon, or helium. Thereafter, the support can be used without calcination or further treatment. Alternatively after drying, the support can be calcined in air or an inert gas, such as nitrogen or helium, to a temperature between about 100xc2x0 C. and about 800xc2x0 C., preferably between about 100xc2x0 C. and about 650xc2x0 C.
An alternate method of deposition of the titanium is from the vapor phase. Volatile titanium compounds, such as titanium chloride, titanium propoxide, or titanium isopropoxide, can be carried through the support in a flow of an inert gas such as nitrogen, argon, or helium. The titanium compound can be heated to volatilize or vaporize it into the inert gas stream. The support can be heated during the process. Thereafter, the support can be used without calcination or further treatment. Alternatively, the support can be calcined in air or an inert gas, such as nitrogen or helium, to a temperature between about 100xc2x0 C. and about 800xc2x0 C., preferably between about 100xc2x0 C. and about 650xc2x0 C.
e. Titanium Dispersed on Promoter Metal Silicates
Yet another suitable support for the catalyst of this invention comprises titanium dispersed on promoter metal silicates. Stoichiometric and non-stoichiometric compounds comprising promoter metal silicates can be used. Any amorphous or crystalline promoter metal silicate is suitably employed. Preferred promoter metal silicates include the silicates of Group 1, Group 2, the lanthanide rare earths, and the actinide metals, and combinations thereof. Non-limiting examples of preferred promoter metal silicates include sodium containing silicate, cesium containing silicate, magnesium silicate, calcium silicate, barium silicate, erbium silicate, and lutetium silicate. The titanium can be dispersed on the promoter metal silicate in a manner analogous to that described in section (d) hereinabove. Analytical methods such as those described in section (d) hereinabove can be used to identify the dispersed titanium phase.
f. Mixtures of Supports
Any combination or mixture of the supports a-e, described hereinabove, can be employed in the catalyst of this invention.
The silver loading on the titanium-containing supports (a-f) can be any which gives rise to the catalyst of this invention. The silver can be added either before, simultaneously with, or after the titanium is added to the support. Generally, the silver loading is greater. than about 0.01, preferably, greater than about 0.02 weight percent, based on the total weight of the catalyst composition. Generally, the silver loading is less than about 20, preferably, less than about 15 weight percent.
The silver component can be deposited or supported on the support by any method known in the art which provides for an active and selective epoxidation catalyst in the process of this invention. Non-limiting examples of known deposition methods include impregnation, ion-exchange, and deposition by precipitation. A preferred method involves contacting the support with a solution of a soluble silver compound. Aqueous and non-aqueous solutions can be employed. The preparation can be done in the presence of light or in the dark. Then, the composite is calcined and optionally reduced to form the catalyst of the invention.
For aqueous solutions, any water soluble silver compound can be used including silver nitrate and silver carboxylates, such as silver oxalate and silver lactate. For non-aqueous solutions of common organic solvents, any soluble silver complex, such as a silver amine complex, can be used. Typically, the molarity of the soluble silver compound ranges from about 0.001 M to the saturation point of the soluble silver compound, preferably, from about 0.005 M to about 0.5 M. The desired quantity of support is added to the solution, and the mixture is stirred under air at a temperature between about 20xc2x0 C. and about 80xc2x0 C. for a time ranging from about 1 hour to about 24 hours. At the end of this period, the solids are either recovered or dried. The solids are not washed, or optionally lightly washed with water, the water optionally containing one or more promoter salts. Thereafter, the composite is dried at a temperature between about 80xc2x0 C. and about 120xc2x0 C., and then calcined, in the presence of oxygen at a temperature between about 200xc2x0 C. and about 800xc2x0 C., preferably from about 350xc2x0 C. to about 750xc2x0 C., for a time from about 1 to about 24 hours. The calcination may be used to decompose the anion of the silver salt, such as the nitrate or lactate. Optionally, the calcined material may be reduced with a liquid or gas phase reducing agent, such as hydrogen, ammonia, or hydrazine, at a temperature between about 20xc2x0 C. and about 500xc2x0 C., preferably between about 100xc2x0 C. and about 400xc2x0 C., for a time from about 1 to about 24 hours to form the catalyst of this invention.
As noted hereinbefore, in one preferred embodiment the catalyst comprising silver and the titanium containing support is essentially free of Group 8 metals, including iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum. The term xe2x80x9cessentially free,xe2x80x9d as used in these context, means that the total concentration of these metals is less than about 0.01 weight percent, preferably, less than about 0.005 weight percent, based on the weight of the total catalyst composition.
In another preferred embodiment, the catalyst comprising silver on the titanium-containing support further comprises gold. Preferably, this catalyst also is essentially free of the Group 8 metals, as noted hereinbefore. Generally, the gold loading is greater than about 0 weight percent, preferably, greater than about 0.01 weight percent, based on the total weight of the catalyst composition. Generally, the gold loading is less than about 20 weight percent, preferably, less than about 10 weight percent. The gold present may be in any oxidation state. The gold may be present as an alloy with the silver.
The gold can be deposited onto the titanium-containing support simultaneously with the silver, or alternatively, in a separate deposition step either before or after silver is deposited. The gold component can be deposited or supported on the support by any method known in the art which provides for an active and selective epoxidation catalyst in the process of this invention. Non-limiting examples of known deposition methods include impregnation, ion-exchange, and deposition by precipitation. A preferred deposition method is disclosed by S. Tsubota, M. Haruta; T. Kobayashi, A. Ueda, and Y. Nakahara, xe2x80x9cPreparation of Highly Dispersed Gold on Titanium and Magnesium Oxide,xe2x80x9d in Preparation of Catalysts V, G. Poncelet; P. A. Jacobs, P. Grange, and B. Delmon; eds., Elsevier Science Publishers B.V., Amsterdam, 1991, p. 695ff, incorporated herein by reference. This method involves contacting the support with an aqueous solution of a soluble gold compound at a temperature and pH sufficient to deposit or react the gold compound onto the support. Non-aqueous solutions can also be employed. Thereafter, the silver component can be deposited on the support in the manner described hereinbefore. Thereafter, the composite containing silver and gold is not washed or is lightly washed, with preferably no more than about 100 ml wash liquid per gram composite. Then, the silver-gold composite is calcined under air at a temperature between about 150xc2x0 C. and about 800xc2x0 C. for a time from about 1 to 24 hours. The calcined material may then be heated in a reducing atmosphere, such as hydrogen, or heated in an inert atmosphere, such as nitrogen, at a temperature between about 150xc2x0 C. and about 800xc2x0 C. for a time from about 1 to 24 hours.
For aqueous solutions, any water soluble gold compound can be used, such as chloroauric acid, sodium chloroaurate, potassium chloroaurate, gold cyanide, potassium gold cyanide, and diethylamine auric acid trichloride. Typically, the molarity of the soluble gold compound ranges from about 0.001 M to the saturation point of the soluble gold compound, preferably, from about 0.005 M to about 0.5 M. The pH of the aqueous gold solution may be adjusted to between about 2 and about 11, preferably, between about 6 and about 9, with any suitable base, such as Group 1 metal hydroxides or carbonates, preferably sodium hydroxide, sodium carbonate, potassium carbonate, cesium hydroxide, and cesium carbonate: The desired quantity of support is added to the solution, or vice versa; and if necessary, the pH is again adjusted. Thereafter, the mixture is stirred under air at a temperature between about 20xc2x0 C. and about 80xc2x0 C. for a time ranging from about 1 hour to about 24 hours. At the end of this period, the solids are recovered, optionally washed with water, the water optionally containing one or more promoter metal salts preferably at a pH between about 5 and about 11. Thereafter, the solids are dried under air at a temperature between about 80xc2x0 C. and about 120xc2x0 C. Afterwards, the solids are treated with a solution containing a silver compound in the manner described hereinbefore. The silver-gold-support composite is calcined under air at a temperature between about 150xc2x0 C. and about 800xc2x0 C. for a time from about 1 to 24 hours. The calcined material may then be heated in a reducing atmosphere, such as hydrogen, or heated in an inert atmosphere, such as nitrogen, at a temperature between about 150xc2x0 C. and about 800xc2x0 C. for a time from about 1 to 24 hours.
In another preferred embodiment, the catalyst comprising silver and titanium or the catalyst comprising silver, gold, and a titanium-containing support contains one or more promoter. elements. Any metal ion having a valence between +1 and +7 which enhances the productivity of the catalyst in the oxidation process can be employed as a promoter element. Factors contributing to increased productivity of the catalyst include increased conversion of the olefin, increased selectivity to the olefin oxide, decreased production of water, and increased catalyst lifetime. Preferred promoter elements include the elements of Groups 1 and 2 of the Periodic Table of the Elements, as well as zinc, cadmium, the rare earth lanthanides, and actinides, as referenced in the CRC Handbook of Chemistry and Physics, 75th ed., CRC Press, 1994. Group 1 elements include lithium, sodium, potassium, rubidium, and cesium; Group 2 elements include beryllium, magnesium, calcium, strontium, and barium. The lanthanide rare earth elements include cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and lutetium. The actinide elements specifically include for the purposes of this invention thorium and uranium. More preferably, the promoter element is sodium, cesium, magnesium, calcium, barium, lanthanum, praseodymium, erbium, or lutetium. Even more preferably, the catalyst contains one or more promoter elements with the proviso that when a lanthanide element is used, it is in combination with a Group 1 and/or a Group 2 element. Most preferably, the catalyst contains a combination of at least one Group 1 element with a Group 2 element and/or a lanthanide element.
If one or more promoter elements are used, then the total quantity of promoter element(s) deposited on the support typically is greater than about 0.001 weight percent, and preferably, greater than about 0.01 weight percent, based on the total weight of the catalyst composition. The total quantity of promoter element(s) deposited on the support is generally less than about 40, preferably, less than about 20 weight percent, based on the total weight of the catalyst. Those skilled in the art will recognize that when a promoter metal titanate or silicate is employed, the weight percentage of promoter metal may be much higher, for example, as high as about 80 weight percent.
The promoter element(s) can be deposited onto the titanium-containing support simultaneously with the silver, or alternatively, in a separate deposition step either before or after silver is deposited. When gold is in the preparation, the promoter element(s) can be deposited onto the titanium-containing support simultaneously with the silver and/or gold, or alternatively, in a separate deposition step either before or after silver and/or gold are deposited. Alternatively, the promoter element can be deposited onto a precursor form of the catalyst before the titanium is added, or after it is added, or simultaneously with the titanium. Typically, the promoter element is deposited from an aqueous or organic solution containing a soluble promoter metal salt. Any salt of the promoter metal with adequate solubility can be used; for example, the metal nitrates, carboxylates, and halides, preferably, the nitrates, are suitable. If an organic solvent is employed, it can be any of a variety of known organic solvents, including, for example, alcohols, esters, ketones, and aliphatic and aromatic hydrocarbons. Ordinarily, the support is contacted with the solution of the promoter metal salt under conditions which are similar to those used for contacting the support with the silver solution. After the promoter metal is deposited, washing is optional, and if done to excess, can leach at least a portion of the promoter element out of the catalyst. Afterwards, calcination under air and optionally reduction with a reducing agent are conducted in a manner similar to. that described hereinabove for the silver deposition.
Optionally, the catalyst of this invention can be extruded with, bound to, or supported on a second support, such as silica, alumina, an aluminosilicate, magnesia, titania, carbon, or mixtures thereof. The second support may function to improve the physical properties of the catalyst, such as, its strength or attrition resistance, or to bind the catalyst particles together. Generally, the quantity of second support ranges from about 0 to about 95 weight percent, based on the combined weight of the catalyst and second support.
The process of this invention can be conducted in a reactor of any conventional design suitable for gas or liquid phase processes. These designs broadly include batch, fixed-bed, transport bed, fluidized bed, moving bed, shell and tube, and trickle bed reactors, as well as continuous and intermittent flow and swing reactor designs. Preferably, the process is conducted in the gas phase and the reactor is designed with heat transfer features for the removal of the heat produced. Preferred reactors designed for these purposes include, fluidized bed and moving bed reactors, as .well as swing reactors constructed from a plurality of catalyst beds, connected in parallel and used in an alternating fashion.
The process conditions for the direct oxidation described herein can vary considerably over a nonflammable and flammable regime. It is beneficial, however, to recognize the conditions which distinguish between nonflammable and flammable mixtures of the olefin, hydrogen, and oxygen. Accordingly, a diagram can be constructed or consulted which for any given process temperature and pressure shows the flammable and non-flammable range of reactant compositions, including the diluent, if used. The more preferred reactant mixtures specified hereinabove are believed to lie outside the flammable regime when the process is operated at the more, preferred temperatures and pressures specified hereinbelow. Nevertheless, operation within the flammable regime is possible, as designed by one skilled in the art.
Usually, the process is conducted at a temperature which is greater than about ambient, taken as 20xc2x0 C., preferably, greater than about 70xc2x0 C. Usually, the process is conducted at a temperature less than about 250xc2x0 C., preferably less than about 225xc2x0 C. Preferably, the pressure ranges from about atmospheric to about 400 psig (2758 kPa).
In flow reactors the residence time of the reactants and the molar ratio of reactants to catalyst will be determined by the space velocity. For a gas phase process the gas hourly space velocity (GHSV) of the olefin can vary over a wide range, but typically is greater than about 10 ml olefin per ml catalyst per hour (hxe2x88x921) preferably greater than about 100 hxe2x88x921, and more preferably, greater than about 1,000 hxe2x88x921. Typically, the GHSV of the olefin is less than about 50,000 hxe2x88x921, preferably, less than about 35,000 hxe2x88x921, and more preferably, less than about 20,000 hxe2x88x921. Likewise, for a liquid phase process the weight hourly space velocity (WHSV) of the olefin component may vary over a wide range, but typically is greater than about 0.01 g olefin per g catalyst per hour (hxe2x88x921), preferably, greater than about 0.05 hxe2x88x921, and more preferably, greater than about 0.1 hxe2x88x921. Typically, the WHSV of the olefin is less than about 100 hxe2x88x921, preferably, less than about 50 hxe2x88x921, and more preferably, less than about 20 hxe2x88x921. The gas and weight hourly space velocities of the oxygen, hydrogen, and diluent components can be determined from the space velocity of the olefin taking into account the relative molar ratios desired.
When an olefin having at least three carbon atoms is contacted with oxygen in the presence of hydrogen and the catalyst described hereinabove, the corresponding olefin oxide (epoxide) is produced in good productivity. The most preferred olefin oxide produced is propylene oxide.
The conversion of olefin in the process of this invention can vary depending upon the specific process conditions employed, including the specific olefin, temperature, pressure, mole ratios, and form of the catalyst. As used herein the term xe2x80x9cconversionxe2x80x9d is defined as the mole percentage of olefin which reacts to form products. Generally, the conversion increases with increasing temperature and pressure and decreases with increasing space velocity. Typically, the olefin conversion is greater than about 0.02 mole percent, and preferably, greater than about 0.10 mole percent, and more preferably, greater than about 0.20 percent.
Likewise, the selectivity to olefin oxide can vary depending upon the specific process conditions employed. As used herein, the term xe2x80x9cselectivityxe2x80x9d is defined as the mole percentage of reacted olefin which forms a particular product, desirably the olefin oxide. Generally, the selectivity to olefin oxide will decrease with increasing temperature and will increase with increasing space velocity. The process of this invention produces olefin oxides in unexpectedly high selectivity. A typical selectivity to olefin. oxide in this process is greater than about 60 mole percent, preferably, greater than about 75 mole percent, and more preferably, greater than about 90 mole percent.
Besides the epoxide formed, water is also formed as a by-product of the process of this invention. Additionally, hydrogen may be reacted directly to form water. Accordingly, it may be desirable to achieve a water/olefin oxide molar ratio as low as possible. In preferred embodiments of this invention, the water/olefin oxide molar ratio is typically greater than about 1/1, but less than about 75/1, and preferably, less than about 50/1, and, more, preferably, less than about 20/1.
When the activity of the catalyst has decreased to an unacceptably low level, the catalyst can be regenerated. Any catalyst regeneration method known to those skilled in the art can be applied to the catalyst of this invention, provided that the catalyst is reactivated for the oxidation process described herein. One regeneration method comprises heating the deactivated catalyst at a temperature between about 150xc2x0 C. and about 500xc2x0 C. in a regeneration gas containing oxygen and optionally an inert gas. In an alternative embodiment, water is beneficially added to the regeneration gas in an amount preferably ranging from about 0.01 to about 100 mole percent.
The invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary of the use of the invention. Other embodiments of the invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention as disclosed herein. Unless otherwise noted, all percentages are given on a mole percent basis.